Exception Handling. Exceptions in Java

Transcription

1 Exception Handling Chapter 9 Exceptions in Java An exception is an object Signals the occurrence of unusual event during program execution Throwing an exception Creating the exception object Handling the exception Code that detects and deals with the exception 1

2 Exceptions in Java Note try block Contains code where something could possibly go wrong If it does go wrong, we throw an exception Note catch block When exception thrown, catch block begins execution Similar to method with parameter Parameter is the thrown object Predefined Exception Classes Java has predefined exception classes within Java Class Library Can place method invocation in try block Follow with catch block for this type of exception Example classes BadStringOperationException ClassNotFoundException IOException NoSuchMethodException 2

3 Predefined Exception Classes Example code Defining Your Own Exception Classes Must be derived class of some predefined exception class Text uses classes derived from class Exception View sample class, listing 9.5 class DivideByZeroException extends Exception View demo program, listing 9.6 class DivideByZeroDemo 3

4 Defining Your Own Exception Classes Note method getmessage defined in exception classes Returns string passed as argument to constructor If no actual parameter used, default message returned The type of an object is the name of the exception class Defining Your Own Exception Classes Guidelines Use the Exception as the base class Define at least two constructors Default, no parameter With String parameter Start constructor definition with call to constructor of base class, using super Do not override inherited getmessage 4

5 Declaring Exceptions Consider method where code throws exception May want to handle immediately May want to delay until something else is done Method that does not catch an exception Notify programmers with throws clause Programmer then given responsibility to handle exception Declaring Exceptions Note syntax for throws clause Note distinction Keyword throw used to throw exception Keyword throws used in method heading to declare an exception 5

6 Declaring Exceptions If a method throws exception and exception not caught inside the method Method ends immediately after exception thrown A throws clause in overriding method Can declare fewer exceptions than declared But not more View program example, listing 9.7 class DoDivision Kinds of Exceptions In most cases, exception is caught In a catch block or Be declared in throws clause But Java has exceptions you do not need to account for Categories of exceptions Checked exceptions Unchecked exceptions 6

7 Kinds of Exceptions Checked exception Must be caught in catch block Or declared in throws clause Unchecked exception Also called run-time Need not be caught in catch block or declared in throws Exceptions that coding problems exist, should be fixed Kinds of Exceptions Examples why unchecked exceptions to are thrown Attempt to use array index out of bounds Null pointer exception Uncaught runtime exception terminates program execution 7

8 Multiple Throws and Catches A try block can throw any number of exceptions of different types Each catch block can catch exceptions of only one type Order of catch blocks matter View example program, listing 9.8 class TwoCatchesDemo Multiple Throws and Catches Exceptions can deal with invalid user input To handle an exception thrown by a method It does not matter where in the method the throw occurs Use of throw statement be should be reserved for cases where it is unavoidable Nested try-catch blocks rarely useful 8

9 The finally Block Possible to add a finally block after sequence of catch blocks Code in finally block executed Whether or not execution thrown Whether or not required catch exists Rethrowing an Exception Legal to throw an exception within a catch block Possible to use contents of String parameter to throw same or different type exception 9

10 Summary An exception is an object derived from class Exception Descendants of class Error behave like exceptions Exception handling allows design of normal cases separate from exceptional situations Two kinds of exceptions Checked and unchecked Summary Exceptions can be thrown by Java statements Methods from class libraries Programmer use of throw statement Method that might throw but not catch an exception should use throws clause Exception is caught in catch block 10

11 Summary A try block followed by one or mor catch blocks More specific exception catch types should come first Every exception type has getmessage method usable to recover description of caught description Do not overuse exceptions 11